This invention relates to a method and device for filtering slurries, and more particularly to a method and a device for filtering slurries by means of a semibatch operation.
In case of filtering used iron oxide in a reactant liquid obtained by a reducing reaction in which iron borings are used as a reducing agent, the filtration of slurries is sometimes required. Some prior methods of filtration which have been employed are: the Nutsche process, the filter press process, the leaf filtration process and the horizontal plate pressure filtration process, etc. However, these processes have been unsatisfactory because the filter cake produced by the filtration is difficult to dispose of; much labor and time are required for disassembly, cleaning and reassembly of the filter; and it is impossible to dispose of the slurry remaining in the filter.
In the light of such circumstances, a method of filtration was developed and put into practical use which comprises the steps of supplying a slurry into a bag-shaped filter medium accommodated in a jacketed tank, filtering the slurry by applying a pressure to it or reducing the pressure outside the bag, supplying a gas under pressure to dispose of the remaining slurry in the filter medium thereby reducing further the liquid content of the filter cake, and then drawing up the bag-shaped filter medium to take out the filter cake. However, the abovementioned method of filtration by using a bag-shaped filter medium is disadvantageous from the viewpoint of its operational efficiency. When a gas is supplied under pressure into the filter medium to dispose of the remaining slurry and reduce further the liquid content of the filter cake, the pressurized gas tends to find its way outside the filter medium by short-circuiting through the side part thereof. This is due to small resistance of filtration on the sides since the slurry tends to remain on the bottom of the bag-shaped filter medium. As a consequence it becomes impossible to dispose of the residual slurry and to reduce the liquid content of the filter cake, thereby making removal of the raw liquid and withdrawal of the cake extremely difficult.